Fired heaters are the most important and largest equipment in any refinery. Refineries always want to push their heaters to the limit. In most of the fired heaters used in the refining and petrochemicals industry, hydrocarbons are heated inside the tubes. As these hydrocarbon fluids are heated, some of the hydrocarbons start cracking and getting converted into carbon due to high film temperature. A major limitation in the ability of refineries to push their heaters to the limit is linked to coke formation in the radiant section tubes. With excessive coke formation, the refiners cannot process the required throughput through the furnace and have to shutdown to clean the heaters prematurely.
In the radiant section of the heaters, heat is transferred from the outside of the furnace tubes to the fluid inside by conduction through the metal of the tubes. Heating the oil molecules to high temperature causes ‘cracking’ with the subsequent formation of coke on the inner surface of the tubes. The coke layer formed is a poor conductor of heat. It insulates the tubes and impedes the heat transfer. The heater must be fired harder to maintain the process fluid at the required outlet temperature.
Firing the heater harder creates inevitable problems. Following initial lay down of coke in a heater tube a vicious circle begins in which more and more coke is laid down. The coking rate increases with the temperature in the tubes. It keeps on depositing in the hotter layers of the existing permeable coke. Eventually the coke hardens and becomes impermeable. The fired heater operation not only becomes inefficient (due to higher heat losses in the flue gases resulting from the increased heat input), but also potentially dangerous. The danger lies in the excessively high temperatures that the tubes may reach, which can cause rapid scaling of the metal and possible rupture of the tube. In addition to actual tube rupture, tubes can sag or bow. A tube will sag under its own weight if the tube becomes grossly overheated. Uneven coke lay down in a tube will also make one side of the tube expand more than the other side, leading to bowing. Tube metals differ according to the type of process and severity of duty but they can all suffer damage due to overheating. Further, as the heater is fired harder, the risk of flame impingement on the tubes also increases. As coke can also be laid down by flame impingement, sometimes localized ‘hot spots’ can develop on tubes where flame impingement has occurred.
Coke formation is thus an inevitable outcome of most fired heater operations. Ultimately the heater needs to be shut down and the coke removed from the tubes. These shutdowns cost refineries a lot of down time and money. A heater which operates in such a way as to slow the rate of coke buildup would be very desirable.
A heater which operates to slow the rate of coke buildup will be most beneficial to the fired heaters in the refinery which are prone to heavy coking. Examples of heaters where there is high need for reduced coking are crude heaters, vacuum heaters, and coker heaters etc.
In the crude heater, crude oil is preheated from 450° F. to 650° F. before it is sent to the atmospheric distillation tower for refining.
In the vacuum heater, bottoms from the atmospheric distillation tower are heated under vacuum to 750-800° F. and then sent to vacuum distillation tower for refining.
In the coker heater, vacuum tower bottoms and residues from other units are sent to the coker heater where they are heated to 900-935° F. Coker heater outlet goes to coke drum and fractionating tower where the products are recovered. Coker heaters have typical run length of only 6 months-1 year.